1. Field of the Invention
The present invention relates to a method and apparatus for measuring an optical property of a fluorescent sample such as a sample treated with a fluorescent whitening agent (hereinafter, also called as “FWA treated sample”), and a printed sample on a substrate treated with a fluorescent whitening agent (hereinafter, also called as “FWA treated substrate”).
2. Description of the Related Art
In recent years, many of the products such as paper and fabrics are treated with a fluorescent whitening agent. It is impossible to evaluate whiteness or color of the products and articles using the products as a substrate, without considering an influence of fluoresced light. Accordingly, it is necessary to solve the problem in technical fields related to these products and articles. Specifically, if UV light which is invisible to human eyes is absorbed by a fluorescent substance, the fluorescent substance is excited, and visible light in a longer wavelength band is radiated from the fluorescent substance. Since a degree of excitation (fluorescent intensity) differs depending on a light source, appearance of an identical measurement sample may differ depending on the light source. In view of the above circumstances, there is a demand for improvement in colorimetry for FWA treated paper and fabric, and printed samples on an FWA treated substrate.
Generally, a visible property i.e. an optical property of a reflecting sample is expressed by a ratio relative to white. Specifically, the optical property of a reflecting sample is expressed based on a total spectral radiance factor B(λ). The total spectral radiance factor B(λ) is a ratio of light emitted from a reflecting sample illuminated in a certain illuminating condition and received in a certain receiving condition to light emitted from a perfect reflecting diffuser in the identical illuminating and receiving conditions at each wavelength λ.
As described above, fluoresced light emitted by excitation light is superimposed over reflecting light on a sample (hereinafter, called as a “fluorescent sample”) such as an FWA treated sample or a printed sample on an FWA treated substrate, and the color of the fluoresced light is observed as an objective light. In other words, radiation from a fluorescent sample is the sum of reflecting light (reflection component) and fluoresced light (fluorescent component) from the fluorescent sample. Accordingly, the total spectral radiance factor B(λ) of a fluorescent sample is given, in the similar manner as described above, as the sum of a reflection spectral radiance factor R(λ) and a fluorescent spectral radiance factor F(λ). The reflection spectral radiance factor R(λ) is a ratio of reflecting light from a fluorescent sample illuminated in a certain illuminating condition and received in a certain receiving condition to light emitted from a perfect reflecting diffuser in the identical illuminating and receiving conditions; and the fluorescent spectral radiance factor F(λ) is a ratio of fluoresced light emitted from the fluorescent sample illuminated in the certain illuminating condition and received in the certain receiving condition to light emitted from the perfect reflecting diffuser in the identical illuminating and receiving conditions. The total spectral radiance factor B(λ) is expressed by the Equation (1).B(λ)=R(λ)+F(λ)  (1)
Since the perfect reflecting diffuser has no fluorescence, and the reflectivity thereof has no dependence on the wavelength of illumination, the total spectral radiance factor B(λ), the reflection spectral radiance factor R(λ), and the fluorescent spectral radiance factor F(λ) are equivalent to the ratios of light of the wavelength λ emitted, reflected and fluoresced from the sample respectively to the illumination of the same wavelength λ except for a proportional coefficient. An object of the colorimetry is to obtain a measurement value analogous to visual observation. In the case where a fluorescent sample having an objective color is measured, the total spectral radiance factor B(λ) is a measurement value to be obtained, and colorimetric values are derived from the total spectral radiance factor B(λ).
CIE (International Committee of Illumination) defines spectral distributions (spectral intensities) of illumination for colorimetry such as Illuminant D65 (day light) and Illuminant A (incandescent light source), as well as standard illuminations such as Illuminants D50, D75, F11, and C. For measurement of fluorescent samples, standard illuminations such as Illuminants C and D50 are generally used. The fluorescent characteristics of a fluorescent sample or a fluorescent substance illuminated by the illumination are expressed by a bi-spectral luminescent radiance factor F(μ,λ). The bi-spectral luminescent radiance factor is matrix data showing the intensity of fluoresced light of the wavelength λ excited by excitation light i.e. incident light of the wavelength μ for illuminating a fluorescent sample surface with a unit intensity i.e. by monochromatic light of a unit intensity.
An example of the matrix data is shown in FIG. 8. The matrix data is three-dimensional data, wherein the fluorescent wavelength λ (unit: nm) and the excitation wavelength μ (unit: nm) are defined in x-axis and y-axis, respectively, and the fluorescent intensity is defined in z-axis. As is obvious from the matrix data, a section (e.g. a section where λ is 550 nm) taken along a specific fluorescent wavelength λ represents a spectral excitation efficiency i.e. an excitation efficiency of excitation light for exciting fluoresced light of the wavelength λ at each wavelength. A section (e.g. a section where μ is 450 nm) taken along a specific excitation wavelength μ represents a spectral intensity of fluoresced light excited by an illumination of 450 nm. Accordingly, in a sense, a fluorescent phenomenon is a phenomenon involving a wavelength conversion from the wavelength μ to the wavelength λ. Therefore, a fluorescent spectral radiance factor F(λ) of a fluorescent sample having a bi-spectral luminescent radiance factor F(μ,λ) is expressed by the Equation (2), where the proportional coefficient is neglected, when illuminated by an illumination I having a spectral distribution I (μ).F(λ)=∫F(μ,λ)·I(μ)d μ/I(λ)  (2)
Specifically, the fluorescent spectral radiance factor F(λ) is obtained as the ratio of convolution of the spectral distribution I (μ) of the illumination I and the bi-spectral luminescent radiance factor F(μ,λ) to the spectral distribution I (λ) of the illumination I. The spectral distribution I (λ) of the illumination I is substantially equivalent to reflections from the perfect reflecting diffuser (plane), except for the proportional coefficient. In the Equations throughout the specification, the symbols “·”, “/”, and “∫” represent multiplication, division, and integration, respectively.
As indicated by the Equation (2), the fluorescent spectral radiance factor F(λ) depends on the spectral distribution I (μ) of the illumination I. Accordingly, the total spectral radiance factor B(λ), which is the sum of the fluorescent spectral radiance factor F(λ) and the reflection spectral radiance factor R(λ) which itself has no dependence on the spectral distribution I (μ) of the illumination I also depends on the spectral distribution I (μ). In other words, the total spectral radiance factor B(λ) to be measured based on R(λ) and F(λ), and calorimetric values derived from the total spectral radiance factor B(λ) are different depending on a difference in spectral distribution of illumination with respect to a fluorescent sample.
Accordingly, it is required to specify a spectral distribution of a certain illumination (hereinafter, illumination for use in evaluating an optical property such as F(λ) and B(λ) is called as “test illumination”) in evaluating an optical property of a fluorescent sample. In actual measurement, the spectral distribution of illumination of a measuring apparatus need to match with the spectral distribution of the specific test illumination. However, it is difficult to match the spectral distribution of illumination of a measuring apparatus with the spectral distribution of the specific test illumination, in other words, to obtain an illumination having the same spectral distribution as the spectral distribution of the standard illumination (such as the illuminant D50 or C) generally used as the test illumination.
There is proposed another approach of numerically calculating the fluorescent spectral radiance factor F(λ) or the total spectral radiance factor B(λ), using the Equation (2) by measuring a bi-spectral luminescent radiance factor F(μ,λ) or a bi-spectral radiance factor B(μ,λ), and based on the measured bi-spectral luminescent radiance factor F(μ,λ) or bi-spectral radiance factor B(μ,λ), and a spectral distribution I (λ) of a test illumination given as numerical data. Here, similarly to the bi-spectral luminescent radiance factor F(μ,λ), the bi-spectral radiance factor B(μ,λ) is matrix data showing an intensity of the total emission of the wavelength λ which is the sum of fluoresced light of the wavelength λ and reflecting light by an illumination of the wavelength μ for illuminating a fluorescent sample surface with a unit intensity. The total spectral radiance factor B(μ,λ) is obtained as a ratio of convolution of a spectral distribution I (μ) of an illumination I and a bi-spectral radiance factor B(μ,λ) to I (λ).B(λ)=∫B(μ,λ)·I(μ)d μ/I(λ)  (3)
However, measurement of the bi-spectral luminescent radiance factor F(μ,λ) or the bi-spectral radiance factor B(μ,λ) (hereinafter, the two radiance factors are generically called as “bi-spectral characteristics”; and the fluorescent spectral radiance factor F(λ) and the total spectral radiance factor B(λ) to be individually calculated are generically called as “spectral fluorescent characteristics”) requires a complicated and time-consuming bi-spectro-fluorometer e.g. a double monochromator comprising two spectral units, one for illumination and the other for receiving. Accordingly, use of the bi-spectro-fluorometer is not practical. Quality control of products such as FWA treated paper as a representative example of a fluorescent sample is generally performed using one of the following two simplified methods.
(Gaertner and Griesser's Method)
In this section, Gaertner and Griesser's method is described as a first approach. As shown in FIG. 9, a fluorescent sample 601 is placed at a sample aperture 603 of an integrating sphere 602 of an optical property measuring apparatus 600. A light flux 605 emitted from a light source 604 such as a xenon flash lamp having a sufficient spectral intensity in a UV region enters into the integrating sphere 602 through an aperture of the integrating sphere 602. A UV cut filter 606 is inserted at such a position as to partially block the optical path of the light flux 605 to remove a UV component from a part of the light flux 605 which passes through the UV cut filter 606. The degree of insertion of the UV cut filter 606 is adjustable so as to allow adjustment of a ratio (relative UV intensity) of intensity of an illumination in a UV region (excitation region) to that of the illumination in a visible region. Both a part of the light flux 605 that has passed through the UV cut filter 606 and a part of the light flux 605 that has not passed through the UV cut filter 606 enter into the integrating sphere 602 and undergo multiple diffuse reflections within the integrating sphere 602, and form illumination for illuminating the fluorescent sample 601.
A component (radiation component 607) of light emitted in a predetermined direction from the surface of the fluorescent sample 601 illuminated by the illumination enters a sample spectral unit 608 for measuring a spectral distribution Sx(λ) of the radiation component 607. Similarly, a light flux 609 having substantially the same spectral distribution as that of the illumination directly enters a monitoring optical fiber 610 so as to be directed to a monitoring spectral unit 611 for measuring a spectral distribution Mx(λ) of the light flux 609. A computation controller 612 calculates a total spectral radiance factor Bx(λ) based on measurement information on the spectral distribution S(λ) of the radiation component 607 and the spectral distribution Mx(λ) of the light flux 609.
Calibration of the relative UV intensity is performed as follows. Specifically, a fluorescent standard containing a fluorescent substance having excitation-fluorescent characteristics, namely, a bi-spectral luminescent radiance factor close to that of the fluorescent sample 601, and whose calorimetric value (e.g. whiteness WIs defined by the CIE) under a specific test illumination is known is used. The fluorescent standard is placed at the sample aperture 603. Then, a total spectral radiance factor B(λ) is measured by the optical property measuring apparatus 600. Then, the degree of insertion of the UV cut filter 606 is adjusted to match a whiteness WI calculated based on the total spectral radiance factor B(λ) with the known whiteness WIs.
The Gaertner and Griesser's method is mechanically complicated and unreliable, and also requires complicated and time-consuming calibration, in other words, measurements and adjustments of the UV cut filter 606 need to be repeated until the whiteness WI agrees with the known whiteness WIs. Also, the above method has the degree of freedom “1”. Accordingly, it is fundamentally impossible to simultaneously calibrate two or more calorimetric values such as the whiteness WI and Tint value, or perform calibration to match the total spectral radiance factor B(λ) with a total spectral radiance factor Bs(λ) to be obtained in the case where a fluorescent sample is illuminated by a known test illumination.
(Method of JP Hei 8-313349A Corresponding to U.S. Pat. No. 5,636,015)
In this section, the method recited in JP Hei 8-313349A corresponding to U.S. Pat. No. 5,636,015 (D1) is described as a second approach. The measurement method recited in D1 is substantially the same, in principle, as the Gaertner and Griesser's method in that illuminations are combined depending on a degree of insertion of the UV cut filter 606 to numerically synthesize the total spectral radiance factor Bx(λ). The degree of freedom is also “1” in D1. The measurement method in D1 is different from the Gaertner and Griesser's method in that the measurement method in D1 comprises adjusting a relative UV intensity at each wavelength λ, numerically synthesizing the total spectral radiance factor B(λ) first, and synthesizing an illumination that gives the total spectral radiance factor B(λ) as a result.
More specifically, an optical property measuring apparatus 700 shown in FIG. 10 is provided with an integrating sphere 702, a first illuminator 704 for emitting a light flux 703 having a UV intensity, a second illuminator 706 for emitting a light flux 705 having no UV intensity, a sample spectral unit 709 for measuring a spectral distribution of light (radiation component 708) emitted from a fluorescent sample 701 placed at a sample aperture 707, a monitoring spectral unit 712 for measuring a spectral distribution of a light flux 710 of illuminations through an optical fiber 711, and a computation controller 713.
In the optical property measuring apparatus 700, the fluorescent sample 701 is illuminated by the first and the second illuminators 704 and 706, and spectral distributions Sx1(λ) and Sx2(λ) of radiations from the fluorescent sample 701, and spectral distributions Mx1(λ) and Mx2(λ) of illuminations are respectively measured. Total spectral radiance factors Bx1(λ) and Bx2(λ) of the fluorescent sample 701 illuminated by the illuminations from the first and the second illuminators 704 and 706 are obtained based on the spectral distributions Sx1(λ) and Sx2(λ) of radiations, and the spectral distributions Mx1(λ) and Mx2(λ) of illuminations. Thereafter, a total spectral radiance factor Bxc(λ) is obtained by linearly combining the total spectral radiance factors Bx1(λ) and Bx2(λ) weighted with a weighting factor W(λ) (hereinafter, also called as a “weight”) stored in advance at each wavelength, as expressed by the Equation (4).Bxc(λ)=W(λ)·Bx1(λ)+(1−W(λ))·Bx2(λ)  (4)The total spectral radiance factor Bxc(λ) is defined as a total spectral radiance factor of the fluorescent sample 701 illuminated by the test illumination.
Similarly to the Gaertner and Griesser's method, the weighting factor W(λ) has a fluorescent characteristic close to a fluorescent characteristic of the fluorescent sample 701, and is determined using a fluorescent standard having a known total spectral radiance factor BS(λ) when illuminated by a test illumination. Specifically, the weighting factor W(λ) is numerically calculated at each wavelength by matching a value of (W(λ)·B1(λ)+(1−W(λ))·B2(λ)) obtained by linearly combining a total spectral radiance factor B1(λ) measured by illuminating the fluorescent standard by the first illuminator 704, and a total spectral radiance factor B2(λ) measured by illuminating the fluorescent standard by the second illuminator 706, with the weighting factor W(λ), with the known total spectral radiance factor Bs(λ) (see e.g. FIG. 2 in D1).
The above method is substantially equivalent to numerically calibrating the relative UV intensity by the Gaertner and Griesser's method, using the total spectral radiance factor B(λ) as a parameter, at each wavelength. Since the method is directed to calibrating the total spectral radiance factor B(λ), the method has an advantage that all the calorimetric values derived from the total spectral radiance factor B(λ) are calibrated. The above method eliminates many shortcomings of the Gaertner and Griesser's method, because an operation of a mechanical movable member, and a cumbersome adjustment of an insertion degree of the UV cut filter 606 in measurement are not necessary. However, both of the first and the second approaches still require a fluorescent standard, and calibration prior to measurement, using the fluorescent standard. Therefore, error resulting from displacement of a light source after calibration is unavoidable. Also, since a fluorescent substance contained in the fluorescent standard is composed of an organic material, the fluorescent standard needs to be replaced about once a month, in view of deterioration of the fluorescent standard.
In view of the above, the inventor of the present application proposed another approach, as disclosed in JP 2006-292510 corresponding to US 2006-227319A1 (D2). In the measurement method in D2, a fluorescent sample “x” having bi-spectral characteristics close to specific bi-spectral characteristics expressed by a predetermined bi-spectral luminescent radiance factor is illuminated by two illuminations I1 and I2 both having a spectral intensity in a visible region, and different relative intensities between an excitation region and a fluorescent region. Then, total spectral radiance factors Bx1(λ) and Bx2(λ) are measured, and spectral distributions I1(μ) and I2(μ) of the illuminations I1 and I2 are measured. Then, as shown by the Equations (5) through (7), fluorescent spectral radiance factors F1(λ), F2(λ), and Fs(λ) by the illuminations I1, I2, and a specific test illumination Is are numerically calculated, using the measured spectral distributions I1(μ) and I2(μ) of the illuminations I1 and I2, a spectral distribution Is(μ) of the test illumination Is which is given as data in advance, and the aforementioned predetermined bi-spectral luminescent radiance factor F(μ,λ)F1(λ)=∫F(μ,λ)·I1(μ)dμ/I1(λ)  (5)F2(λ)=∫F(μ,λ)·I2(μ)dμ/I2(λ)  (6)Fs(λ)=∫F(μ,λ)·Is(μ)d μ/Is(λ)  (7)
Here, the weighting factors W(λ) and 1−W(λ) are determined so that the fluorescent spectral radiance factor Fs(λ) by the test illumination Is is expressed by a weighted linear combination obtained by linearly combining the fluorescent spectral radiance factors F1(λ) and F2(λ) by the illuminations I1 and I2, weighted with the weighting factors W(λ) and 1−W(λ), as expressed by the Equation (8).Fs(λ)=W(λ)·F1(λ)+(1−W(λ))·F2(λ)  (8)
As expressed by the Equation (1), the total spectral radiance factor B(λ) is the sum of the fluorescent spectral radiance factor F(λ), and the reflection spectral radiance factor R(λ) which has no dependence on a spectral distribution of illumination. Accordingly, the weighting factors W(λ) and 1−W(λ) expressed in the Equation (8) can be also applied to the measured total spectral radiance factors Bx1(λ) and Bx2(λ). Thereby, a total spectral radiance factor Bxc(λ) close to a total spectral radiance factor Bxs(λ) of the fluorescent sample illuminated by the specific test illumination can be calculated, using the Equation (9).Bxc(λ)=W(λ)·Bx1(λ)+(1−W(λ))·Bx2(λ)  (9)
In the above approach, the weighting factors W(λ) and 1−W(λ) are determined based on the fluorescent spectral radiance factor F(λ) using the bi-spectral luminescent radiance factor F(μ,λ). Alternatively, the weighting factors W(λ) and 1−W(λ) may be determined based on a total spectral radiance factor B(λ) using a bi-spectral radiance factor B(μ,λ).
The method of D2 is advantageous in measuring an optical property of FWA treated paper or a printed sample on FWA treated paper, without using a fluorescent standard or performing calibration using a fluorescent standard. However, similarly to the Gaertner and Griesser's method and the method of D1, which are performed based on a premise that excitation-fluorescent characteristics are close to each other between a fluorescent sample and a fluorescent standard, the method of D2 is performed based on a premise that bi-spectral characteristics (in the following section corresponding to a bi-spectral luminescent radiance factor F(μ,λ)) of FWA treated paper as a sample are close to a predetermined bi-spectral luminescent radiance factor to be used in calculation. Therefore, if the bi-spectral luminescent radiance factor F(μ,λ) is not close to the predetermined bi-spectral luminescent radiance factor to be used in calculation, an error may be increased. As described above, since measurement of a bi-spectral luminescent radiance factor F(μ,λ) of a sample requires a bi-spectro-fluorometer, the measurement is generally difficult. As practical means equivalent to the method using a bi-spectro-fluorometer, there is proposed an idea of allowing a user to select a bi-spectral luminescent radiance factor close to the bi-spectral luminescent radiance factor F(μ,λ) of a sample, out of multiple bi-spectral luminescent radiance factors stored in advance. However, letting a user to select a bi-spectral luminescent radiance factor imparts a load to the user. Also, since the bi-spectral characteristics of samples are generally not disclosed to the public, the probability of error by erroneous selection is likely to be increased.